题目

 Never before ______ anyone in Chinese history who can play tennis as well as Li Na. A. has been there       B. there has been                       C. has there been       D. there been has 答案：C交警常用“司机饮酒检测仪”检查司机是否酒后驾车，因为酒中的乙醇分子可以使橙红色的重铬酸钾变为绿色的硫酸铬．硫酸铬[Cr2（SO4）3]中Cr元素的化合价为（　　）A．+3B．+5C．+6D．+7